non-isolated buck converters and MCPCB

Hello,

We have designed and tested a number of small (10-20W) non-isolated buck converters for us in LED lighting products. They work well, and have proven exceptionally efficient.

Though some serious heavy weights in this area, such as Ron Lenk and Steve Winder, have developed such supplies for use in LED products, I'm left slightly with the jitters.

It is a new area for us, as much of our team is in Korea, where they simply don't do non-isolated supplies, as they are apparently not allowed.

The potential liability issues are huge, not to mention killing people is usually considered a bad thing for a product.

Given that LED products, ours included, are built on MCPCB materials (aluminum), you don't have 1.5mm of fiberglass to isolate the PCB board from the metal housing of the fixture.

The isolation of the mains voltage from the case, and thus from the end user, boils down to the dielectric material of the PCB.

From our knowledge, these materials are something like 1.3kV/mil.

At 3-5 mills for the thermally conductive dielectric materials commonly used in the industry, this would seem to imply a good 3.5kV or more for breakdown voltage.

Anyway, the simple question is can such supplies really be considered safe in this type of application?

Much better designers than I seem to think so, but would really love to hear the thoughts of others on this.

Thanks!

Hi mathos00 , i also design LED product with non isolate solution.this is nightmare if use with MCPCB because the MCPCB can easily failed Hi-Pot test 3.6kV.
you can try Thermal insulator with high breakdown voltage and put between mcpcb and casing body.try avoid using metal screw.also creepage distance must be > 6.4mm. and then you need to test with Hi-Pot test 4kV,100mA with 60 seconds.if this pass,your product can be consider safe.

Hi inifinitrix

Thanks for the feedback.

Certainly the plastic screws are a good idea.

We'll probably also be using 3-5mil thermal pad or tape under the MCPCB instead of grease, such as something from 3M (info here: https://multimedia.3m.com/mws/media...TSeSSSSSS--&fn=ElecAssmbly_6001292_lowres.pdf )

We're on 120vac here in N. America, so wondering if 6.4mm is not a bit much on the required creepage distance?

there is nylon screw that you can use.
i saw some led lamp manufacture use ceramic heatsink to isolate also.
in your case,120vac should be easier,our design mostly target for 240vac...
your Hi-Pot voltage also not so high as 3.6~3.7Kv, and creepage distance also less,i'm not sure what is the creepage for 120vac..